Is there anything symbolic or ironic in The Secret Life of Walter Mitty?
There's a lot of irony in the story. Irony is traditionally defined in modern literature as "the technique of indicating an intention or attitude opposed to what is actually stated." (http://www.enotes.com/literary-terms/irony). Just about every fantasy Walter Mitty has is irony. His attitude in the fantasy is one of decisiveness while in real life he allows his wife to order him around. In the fantasies he intentionally makes himself the center of attention whether as the captain or on the witness stand, and yet in real life he wants to avoid attention, and when others do pay attention to him, like the person on the street who laughed about him saying "puppy biscuits", it's for ridicule. It's ironic that a man who wants to be so strong and commanding (and who in his fantasies *sees* himself as strong and commanding) is such a wimp.
The root of the term "irony" comes from ancient Greek theater, where a stock character, called the "eiron" played a foolish overlooked character. The "alazon" would be a character who brags or schemes to take advantage of the town or whatever. The alazon would think the eiron was too foolish or insignificant to worry about. Ultimately the eiron would foil the plans of the alazon. Thus, the least likely character would be the bravest, most clever etc.....
This fits all of the fantasies that Walter enjoys. His life is completely unimportant and he is powerless to change it. So imagines ironic, important endings to his sad trivial life.